Copper metallization is increasingly being used for advanced semiconductor device integrated circuit fabrication including semiconductor features having sub-quarter micron linewidths and high aspect ratios to larger features such as bonding pads. Copper and its alloys have lower resistivity and better electromigration resistance compared to other metals such as, for example, aluminum. These characteristics are critical for achieving device reliability together with higher current densities and increased signal propagation speed. While several processing difficulties related to forming copper semiconductor features have been overcome, several problems remain, especially in the areas of line to line current leakage between copper interconnects, an increased tendency of copper to electro-migrate through low-K dielectric insulating layers, and dielectric breakdown of low-K dielectric insulating layers.
One problem affecting copper metallization is the tendency of copper to easily form oxides of copper, for example CuO or CuO2, upon exposure to oxidizing environments including humid environments. According to prior art processes, following the copper CMP process the exposed copper is protected by depositing overlying capping layers which may also function as an etch stop layer in formation of an overlying level of copper interconnects. For example, to form the next level of the device, a metal nitride layer which functions as an etching stop layer in formation of metal interconnect features such as vias or dual damascenes in overlying dielectric insulating layers, is typically deposited over the exposed copper following a CMP process. The overlying etching stop layer is also intended to act to prevent further copper oxidation and to reduce electro-migration of copper.
The dual goals of preventing copper electro-migration and preventing cross-interconnect current leakage have not been adequately solved for several reasons. For example, porous silicon oxide based low-K dielectric insulating layers having an interconnecting porous structure have exhibited reduced adhesion to overlying layers, for example etch stop layers, and have increased the tendency of integrated circuit damascene features, such as copper interconnects, to exhibit increased current leakage and electro-migration of copper ions. For example, a phenomenon known as time dependent dielectric breakdown (TDDB) is believed to result from charge accumulation due to slow current leakage over time along micro-cracks developed along poorly adhering material interfaces, for example poor adhesion of an overlying etch stop (capping) layer to an underlying inter-metal dielectric (IMD) layer. As a result, electrical performance and device reliability of semiconductor devices is compromised.
Thus, there is a continuing need for novel semiconductor integrated circuit manufacturing methods to improve the electrical performance of copper interconnect features including methods to improve adhesion of overlying capping layers.
It is therefore an object of the invention to provide a method for forming a copper interconnect capping layer to improve the electrical performance of copper interconnect features including methods to improve adhesion, in addition to overcoming other shortcomings of the prior art.